Method for the production of azo compounds

ABSTRACT

The invention relates to a method for production of azo compounds. In particular the invention relates to a step-wise method for production of an azo compound, by seeding and oxidation of the corresponding hydrazoic compound.

FIELD OF THE INVENTION

The present invention relates to a method of preparation of azocompounds.

BACKGROUND OF THE INVENTION

Azo compounds, and notably 2,2′-azobis(isobutyronitrile), are well-knownproducts that are used notably as a blowing agent or synthesisintermediate or initiator of polymerization reactions employing freeradicals.

These reactions can be reactions of bulk polymerization, solutionpolymerization, suspension polymerization or emulsion polymerization,and make use of a great variety of monomers, for example (meth)acrylicmonomers, vinylic monomers such as acrylamide, acrylonitrile, alkyl(meth)acrylate, styrene, vinyl acetate and chloride, vinylidenechloride. The fields of application are therefore very varied and relatenotably (but not exclusively) to acrylic sheets or fibers, flocculents,paints, coating resins, grafted polyols, polystyrene, PVC, PVA, PMMA.

Azo compounds are generally obtained by oxidation of the correspondinghydrazo derivatives. After oxidation, the resulting suspension isdrained, then dried to give a solid in the form of powder with averagegrain size generally between 20 and 110 μm. For example,2,2′-azobis(isobutyronitrile) obtained by reacting the correspondinghydrazo derivative with chlorine, in an aqueous medium, is in the formof powder with average grain size of about 45 μm after drying and thegrain size at 10 wt. % (d10) is about 20 μm.

Now, azo compounds, in the form described above, pose many problems:

-   -   they generate dust that may present a risk of explosion and/or        an industrial health risk,    -   they have poor castability    -   and problems of lumping are often encountered during storage.

The problems identified above are solved partly or completely by theinvention. The invention supplies a global solution that makes itpossible to increase the average grain size of an azo compound relativeto that of a compound obtained by a conventional method of manufacture.The average grain size can be doubled or even trebled.

SUMMARY OF THE INVENTION

The invention therefore provides a method of manufacture of an azocompound (A), starting from the corresponding hydrazo compound (HA), byseeding.

The method according to the invention includes a step in which anoxidizing agent is reacted with a hydrazo compound (HA), in a liquidmedium, in the presence of a sufficient quantity of crystals of thecorresponding azo compound (seeds).

DETAILED DESCRIPTION OF THE INVENTION

According to one embodiment, the hydrazo compound is in suspension inthe liquid medium.

According to one embodiment, the hydrazo compound is in emulsion in theliquid medium.

This oxidation step can be represented schematically by the followingequation:

where R and R′ in (HA) and (A) can be identical or different, and theyeach represent:

-   -   a linear or branched alkyl group, preferably a C₁-C₆ alkyl        group, possibly substituted by a hydroxy, alkoxy or carboxy        group or by a halogen atom, or    -   a cycloalkyl group preferably with 3 to 6 carbon atoms, possibly        substituted by a hydroxy, alkoxy or carboxy group or by a        halogen atom, or    -   an aryl group such as phenyl or naphthyl, possibly substituted        by a hydroxy, alkyl, alkoxy or carboxy group or by a halogen        atom, or    -   an aralkyl group such as benzyl or phenethyl, possibly        substituted by one or more alkyl, alkoxy, hydroxy or carboxy        groups, or by one or more halogen atoms; or alternatively R and        R′ form, with the carbon atom to which it is (or they are)        joined, a cycloalkyl radical.

As examples of compound (A) we may mention 2,2′-azobis isobutyronitrile(R═R′═CH₃), 2,2′-azobis(2,4-dimethyl-valeronitrile),2,2′-azobis(2-methylbutyronitrile),1,1′-azobis(1-cyclohexanecarbonitrile) and 4,4′-azobis(4-cyanopentanoicacid).

As examples of oxidizing agent we may mention chlorine, oxygen, hydrogenperoxide and ozone.

Preferably, the liquid medium is aqueous (i.e. consisting essentially ofwater).

In addition to the reactants (HA and oxidizing agent), the liquidreaction medium can contain a catalyst, for example bromine ions. It canalso contain additives, such as surfactants, for example sodium bissulfosuccinate, notably sodium bis(2-ethylhexyl)-sulfosuccinate.

The temperature of the reaction medium is generally close to roomtemperature. It is preferably between 15 and 25° C. and advantageouslybetween 18 and 20° C.

The seeds or crystals of compound (A) can be present in the reactionmedium before the start of the oxidation reaction and/or can beintroduced during the oxidation reaction.

After the oxidation step, the resulting suspension is drained and thenthe solid obtained is washed and finally dried.

According to a preferred embodiment of the invention, dihydrocitogen(R═R′═CH₃), in suspension in an aqueous medium, is oxidized withchlorine in the presence of a sufficient quantity of crystals of2,2′-azobis(isobutyronitrile) (AIBN) with average grain sizeadvantageously between 110 and 180 μm and better still between 110 and150 μm.

Preferably, these crystals of AIBN (seeds) are present at the start ofthe oxidation step and represent between 0.5 and 20 wt. %,advantageously between 5 and 15 wt. % relative to the reaction medium(i.e. water+reactants+additives+seeds).

The dihydrocitogen suspended in the aqueous medium represents preferablybetween 2 and 30 wt. %, and advantageously between 5-15 wt. % relativeto the reaction medium (i.e. water+reactants+additives+seeds).

The reaction medium can be stirred by any known means.

The temperature of the reaction medium is preferably between 15 and 25°C. and advantageously between 18 and 20° C.

The duration of the oxidation step according to the invention variesdepending on the desired granulometry. It is preferably between 2 and 6hours.

Preferably, the chlorine is injected into the aqueous mediumcontinuously, in gaseous form, throughout the oxidation.

At the end of the oxidation step according to the invention, the productobtained is drained, then washed and finally dried to give AIBN of thedesired average grain size.

The AIBN crystals (seeds) can be prepared starting from dihydrocitogenby a conventional method of oxidation, followed by a step (r1) in whichdihydrocitogen is reacted with chlorine in an aqueous medium containinga proportion of the suspension resulting from the conventional method(conventional method of oxidation).

Depending on the average grain size of AIBN desired, dihydrocitogen canbe reacted again (r2) with chlorine in an aqueous medium containing aproportion of the suspension resulting from the preceding step (r1).

Advantageously, the seeds can be obtained by reacting n times, n beingan integer greater than 2, preferably in the range from 3 to 5,dihydrocitogen with chlorine in an aqueous medium containing each time aproportion of the suspension resulting from the preceding oxidationreaction.

Preferably, the aqueous medium for the reaction step (rn) contains halfthe quantity of the suspension resulting from the reaction step(r(n-1)).

The preparation of seeds of AIBN described above can be applied to otherazo compounds A, oxidizing agents and liquid medium.

When the average grain size of the seeds intended for the oxidation stepaccording to the invention is reached, it is not necessary to proceed asdescribed above for making new seeds. It is sufficient, after eachoxidation batch, to discharge just a proportion of the resultingsuspension and leave the rest of it for the next oxidation step. Thismakes it possible to deliver seeds of intermediate grain size.

It is also possible to use, as seeds, the crystals of compound Aobtained after draining a suspension resulting from the oxidation stepaccording to the invention, followed if necessary by drying.

The applicant found that compound A obtained after the oxidation stepdescribed above facilitates the subsequent operations of draining anddrying and thus makes it possible to increase the productivity of themanufacturing plant. The granulometry of the dried compound A is lessspread out and the dried compound A contains less dust and offers bettercastability.

EXAMPLES Preparation of the Seeds

Test 1

Introduce the following, in succession, into a 1.5-liter reactorequipped with a stirring system that can be used for mixing asuspension:

-   -   216 g (1.3 mol) of dry dihydrocitogen    -   1350 g of water    -   0.1 g of di(2-ethylhexyl)sulfosuccinate (DOS)    -   0.6 g of sodium bromide.

Then introduce continuously, by means of a gas feed pipe submerged inthe reactor, into the aqueous medium thus formed, 95 g of chlorine overa total time of 5.5 h. During the reaction, keep the medium at atemperature between 18 and 20° C. Stop the stirrer one hour after theend of introduction of the chlorine.

Then filter the suspension, and wash the solid obtained until the pH ofthe wash water is close to 7.

Finally dry the washed solid under vacuum at 30° C. for 12 h to attain afinal moisture content below 0.05 wt. %. The dried solid (crystals ofazobis isobutyronitrile (AIBN)) has an average grain size (d50) of 45 μmand a granulometry (d10) of 20 μm. The castability of this solid is verypoor.

Test 2

Follow the procedure described in test 1, but adding 25 g of dried solidresulting from test 1 before introducing the chlorine.

After drying, crystals are obtained with average grain size (d50) of 75μm and with a d10 of 40 μm.

Test 2a

Follow the procedure described in test 2, but using 216 g (instead of25) of AIBN crystals resulting from test 1.

After drying, crystals are obtained with average grain size (d50) of 60μm and with a d10 of 35 μm.

Test 3

Follow the procedure described in test 2, but adding 25 g of dried solidresulting from test 2 instead of the solid resulting from test 1.

After drying, crystals are obtained with average grain size (d50) of 85μm.

Test 4

Follow the procedure described in test 1 and at the end of the reactiononly discharge half of the suspension obtained. Then introducesuccessively into the reactor containing the other half:

-   -   216 g (1.3 mol) of dry dihydrocitogen    -   675 g water    -   0.1 g of di(2-ethylhexyl)sulfosuccinate (DOS)    -   0.3 g of sodium bromide.

Then introduce continuously, by means of a gas feed pipe submerged inthe reactor, into the aqueous medium thus formed, 95 g of chlorine overa total time of 5.5 h. During the reaction, keep the medium at atemperature between 18 and 20° C. Stop the stirrer one hour after theend of introduction of the chlorine, then again discharge only half ofthe suspension obtained.

Then introduce the various constituents of the charge indicated abovesuccessively into the reactor containing the other remaining half. Afterstopping the reaction, discharge only half of the suspension obtainedand introduce, into the reactor containing the other half, the samecharge as for the preceding batch.

After two additional semi-discharges (as indicated above), use half ofthe final suspension for carrying out example 1 and drain the otherhalf, then wash the solid until the pH of the wash water is close to 7,and finally wash the dried solid to give crystals with average grainsize (d50) of 150 μm and with granulometry (d10) of 60 μm.

Example 1

Introduce successively, into the reactor containing half of the finalsuspension resulting from test 4:

-   -   216 g (1.3 mol) of dry dihydrocitogen    -   675 g of water    -   0.1 g of di(2-ethylhexyl)sulfosuccinate (DOS)    -   0.3 g of sodium bromide.

Then introduce continuously, by means of a gas feed pipe submerged inthe reactor, into the aqueous medium thus formed, 95 g of chlorine overa total time of 5.5 h. During the reaction, keep the medium at atemperature between 18 and 20° C. Stop the stirrer one hour after theend of introduction of the chlorine, and filter the suspension. Thenfilter the solid obtained, and wash it with water until the pH of thewash water is close to 7. Relative to test 1, the filtration time isreduced by 50%.

The residual moisture content of the washed solid is 30% lower relativeto that of the solid obtained according to test 1 and the apparentdensity is increased by 30%.

Finally, dry the washed solid under vacuum at 30° C. for 6 h, to reach afinal moisture content below 0.05 wt. %. The dried solid (crystals ofazobis isobutyronitrile (AIBN)) has an average grain size (d50) of 150μm and a granulometry (d10) of 60 μm.

Example 2

Follow the procedure described in example 1 except that the duration ofintroduction of the chlorine is 4 h. A product similar to that ofexample 1 is obtained.

Example 3

Follow the procedure described in example 1 except that the stirringspeed is increased by 50%. A product similar to that of example 1 isobtained.

Example 4

The operating conditions of example 1 were reproduced on an industrialscale in a 5 m³ reactor. A product similar to that of example 1 wasobtained.

Example C1

Follow the procedure described in test 1 except that the chlorine isintroduced over a period of 4 h (instead of 5.5). The dried solid(crystals of azobis isobutyronitrile (AIBN)) has an average grain size(d50) less than that of test 1.

Example C2

Follow the procedure described in test 1 except that the stirring speedis increased by 50%. An average grain size less than that of thecrystals of test 1 is obtained.

Example 5

Follow the procedure described in test 1 except that, before the startof chlorination, 216 g of crystals of AIBN with average grain size of150 μm, obtained in example 1, are introduced into the reactor.

After drying, AIBN crystals are obtained with an average grain size(d50) of 150 μm.

1. Method of manufacture of an azo compound (A) comprising a step in which an oxidizing agent is reacted (in accordance with the equation given below) with a hydrazo compound (HA), in a liquid medium, in the presence of a sufficient quantity of crystals of the corresponding azo compound (seeds):

where r and r′ in (ha) and (a) can be identical or different, with each representing: a linear or branched alkyl group, preferably a C₁-C₆ alkyl group, possibly substituted by a hydroxy, alkoxy or carboxy group or by a halogen atom, or a cycloalkyl group preferably with 3 to 6 carbon atoms, possibly substituted by a hydroxy, alkoxy or carboxy group or by a halogen atom, or an aryl group such as phenyl or naphthyl, possibly substituted by a hydroxy, alkyl, alkoxy or carboxy group or by a halogen atom, or an aralkyl group such as benzyl or phenethyl, possibly substituted by one or more alkyl, alkoxy, hydroxy or carboxy groups, or by one or more halogen atoms; or alternatively R and R′ form, with the carbon atom to which it is (or they are) joined, a cycloalkyl radical.
 2. Method as claimed in claim 1, characterized in that compound A comprises 2,2′ azobis isobutyronitrile (R═R′═CH₃), 2,2′-azobis(2,4-dimethyl-valeronitrile), 2,2′-azobis(2-methylbutyronitrile), 1,1′-azobis(1-cyclohexanecarbonitrile) or 4,4′-azobis(4-cyanopentanoic acid).
 3. Method as claimed in claim 1, characterized in that the oxidizing agent comprises chlorine, oxygen, hydrogen peroxide or ozone.
 4. Method as claimed in claim 1, characterized in that the liquid medium is aqueous.
 5. Method as claimed in claim 1, characterized in that the temperature of the reaction medium is between 15 and 25° C.
 6. Method as claimed in claim 5, characterized in that the temperature of the reaction medium is between 18 and 20° C.
 7. Method as claimed in claim 1, characterized in that after the oxidation step, the resulting suspension is drained then washed and dried.
 8. Method as claimed in claim 1, characterized in that the crystals of compound (A) can be prepared starting from the corresponding compound HA by a conventional method of oxidation followed by n reactions, n being an integer greater than 2, in the course of which compound HA is oxidized in a liquid medium containing a proportion of the suspension resulting from the preceding reaction.
 9. Method of claim 8 wherein n is an integer greater in the range from 3 to
 5. 10. Method of manufacture of 2,2′-azobis(isobutyronitrile) (AIBN) starting from dihydrocitogen, characterized in that dihydrocitogen, in suspension in an aqueous medium, is oxidized with chlorine in the presence of a sufficient quantity of crystals of 2,2′-azobis(isobutyronitrile) with average grain size between 110 and 180 μm.
 11. Method as claimed in claim 10 wherein said average grain size advantageously between 110 and 150 μm.
 12. Method as claimed in claim 10, characterized in that the crystals of AIBN (seeds) are present at the start of the oxidation step and represent between 0.5 and 20 wt. %, relative to the reaction medium.
 13. Method as claimed in claim 12, wherein the crystals of AIBN represent between 5 and 15 wt. %, relative to the reaction medium.
 14. Method as claimed in claim 10, characterized in that the dihydrocitogen suspended in the aqueous medium represents between 2 and 30 wt. % relative to the reaction medium.
 15. Method as claimed in claim 14, characterized in that the dihydrocitogen suspended in the aqueous medium represents between 5 and 15 wt. % relative to the reaction medium.
 16. Method as claimed in claim 10, characterized in that the temperature of the reaction medium is between 15 and 25° C.
 17. Method as claimed in claim 16, characterized in that the temperature of the reaction medium is between 18 and 20° C.
 18. Method as claimed in claim 10, characterized in that after the oxidation step, the resulting suspension is drained then washed and dried.
 19. Method as claimed in claim 10, characterized in that the crystals of compound (A) can be prepared starting from the corresponding compound HA by a conventional method of oxidation followed by n reactions, n being an integer greater than 2, in the course of which compound HA is oxidized in a liquid medium containing a proportion of the suspension resulting from the preceding reaction.
 20. Method of claim 19 wherein n is an integer greater in the range from 3 to
 5. 